Methods for prenatal diagnosis of sickle cell anemia and homozygous beta thalassemia are being developed. These methods have been successfully applied to a pregnancy at risk for homozygous beta thalassemia. We believe that this service is now available to similar pregnancies. The molecular mechanism of the thalassemia syndromes is being investigated. Accurate measurement of globin structural genes shows that the alpha globin structural gene is duplicated, the alpha thalassemia syndromes are due to deletions of the alpha globin structural genes, and the severity of the clinical disease can be correlated to the number of globin genes deleted.